A rechargeable battery or storage battery comprises one or more electrochemical cells. Rechargeable batteries are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from button cells to megawatt systems used to stabilize an electrical distribution network.
Large-scale non-mobile applications for secondary batteries include grid energy storage applications. Grid energy storage applications use rechargeable batteries for load leveling, where they store electric energy for use during peak load periods, and for renewable energy uses, such as for storing power generated from photovoltaic arrays during the day to be used at night.
A sodium-sulfur (NaS) battery is a type of molten metal battery comprising an elemental sodium anode and a sulfur cathode that is used for some grid energy applications.
This battery has a solid electrolyte that uses an operating temperature around 300 to 350 ° C. to lower the electrical resistance through the solid electrolyte for sodium ions traveling back and forth between the anode and the cathode while the battery is in use. Since molten sulfur is not electrically conductive, a porous current collector, such as comprising graphite or a carbon felt matrix, is incorporated to provide a large surface area at which electrons are transferred from/to the sulfur cathode during the charge/discharge of the battery. This battery has a high energy density, high efficiency of charge/discharge (89-92%) and long cycle life, and is fabricated from inexpensive materials. Limitations of this battery include a high operating temperature that is between 300 to 350 ° C., and reliability problems due to the corrosive nature of sulfur.
A so-called “Zebra Battery” is another type of sodium-based battery that is used for grid energy applications. This battery has a solid electrolyte and an operating temperature around 300° C. to lower the resistance through the solid electrolyte for sodium ions. This battery uses molten sodium at the anode and a solid metal compound cathode comprising NiCl2 or FeCl2. The theoretical specific energy of this battery is slightly higher than that of the NaS battery (790 Wh/kg vs. 760 Wh/kg). The Zebra battery generally provides high cell voltage, high reliability, and adequate pulse power.
Special safety precautions and thermal management required for NaS and Zebra batteries prevent the utilization of the high energy densities theoretically possible. The high temperature (300 to 350° C.) required for operation of these sodium-based batteries also limits the selection of materials. Further, the required high temperature operation necessitates a cylindrical cell design due to high operating pressure, which reduces the packing density of such batteries compared to planar (e.g., rectangular) designs.